1. Field of the Invention
The present invention relates to an internal combustion engine having dual tract induction, that is to say two tracts leading to each intake valve of the engine.
2. Disclosure Information
In racing engines not intended for road vehicles, the intake ports have been inclined as near as possible to the valve axis. This makes better use of the skirt area of the valve opening and facilitates a ram effect to maximise the intake charge. However, this is not practicable in a commercial engine as the height of the engine is restricted and one is obliged to bend the intake port to permit sensible packaging. This bending distorts the flow conditions and introduces an asymmetry causing the flow to cross the valve stem which introduces turbulence and reduces the usage of the available skirt area.
GB-971,211 provides two intake tracts leading to each inlet valve from above with the aim of restoring symmetry and improving the breathing so as to improve top end performance. Other known designs (see GB-728,487) have two intake tracts, one opening on to the side of the engine in the conventional manner and the other on to the top surface of the engine. A further prior art proposal has been to provide tracts from each side of the engine leading to the same intake port.
With tracts entering the cylinder head from both sides of the engine, the performance improvement obtained was found disappointing and development was discontinued. In the case of engines with one or more tracts opening on to the top of the cylinder head, a complex design of inlet manifold is required to straddle the valve train. Furthermore, the engine needs to be fuel injected as the problems caused by the wet manifold of a carburetted engine would be difficult to overcome on account of the complex manifold design.
In further known proposals, for example GB-1,179,087, U.S. Pat. No. 4,550,699, U.S. Pat. Nos. 4,174,686 and GB-1,567,812, two inlet tracts have been used with the intention of creating swirl in the intake charge. The above patents include proposals for angling the tracts tangentially relative to the valve skirt and for introducing different amounts of air through the two tracts in order to promote swirl. The present invention, on the other hand, is not concerned with promoting swirl and on the contrary takes steps to avoid such swirl.
According to the present invention, there is provided an internal combustion engine including a cylinder head, intake poppet valves and intake tracts controlled by said valves and leading from a side face of the cylinder head to the engine cylinders, there being two tracts leading to each intake valve, characterised in that the tracts are dimensioned and shaped for substantially equal air flow under all engine operating conditions so as to supply air uniformly to the perimeter of the valve skirt without introducing swirl into the intake charge, the tracts being formed of outer sections which extend substantially parallel to the base of the head and which open separately on to the said side face of the engine and inner sections which lie in a plane containing the axis of the valve and disposed symmetrically within the latter plane about the axis of the valve, the inner sections of the tracts remaining symmetrical about the valve axis for a sufficient length to ensure that the major component of the velocity of the intake charge is directed along the valve axis.
In the prior art in which the purpose of the separate porting was to promote swirl, it was possible for the ports to open onto the side of the engine. However, it was never suggested in the prior art to employ side entry porting in arrangements where laminar air flow was to be maintained as such a port geometry appears to negate the purpose of the design, namely to maintain symmetry and equal air flow around the entire valve skirt perimeter. The present invention is predicated on the realisation that by the use of side entry, an engine with non-swirl twin porting can be packaged to fit under the bonnet of a modern motor vehicle without significantly impairing its operating performance.
Preferably, the second sections are inclined at an angle of less that 45.degree. to the valve aXis so that the major component of the velocity of the intake charge should be directed along the valve axis.
The plane containing the axes of the inner sections of the tracts should preferably extend along the engine centre line so that the tracts should be of equal length but if insufficient space is available for two port section between adjacent intake valves then it is possible for the planes of the inner tract sections to be inclined to the engine centre line.
It is also convenient that the valve axes should be inclined away from the engine centre line as this then calls for a less abrupt transition from the outer section to inner section of each tract.
The invention is concerned with an engine primarily designed for a road vehicle and differs from all prior art proposals employing dual tract induction in that the design of the intake porting is intended to take advantage not only of the top end performance but of the lean burn capability and of the improved exhaust emissions. These improvements have not previously been noted because no attempt was made to assess the performance of the prior art proposals under part load and low speed conditions as the engines were not designed for this purpose. It is believed that this is the reason why the prior art proposals have hitherto remained experimental and have never been incorporated in a commercial engine.
It has now been found, very surprisingly, that non-swirl dual tract induction can achieve significant improvements under low load and part load operation. The surprising nature of this discovery is emphasised by the fact that an engine with twin tract induction has been made to run with leaner mixtures than have previously been found possible even using four valves per cylinder.
It is generally well known that lean burn is enhanced by introducing turbulence into the charge and with this aim in mind, cylinder heads designed for lean burn have generally included means for increasing swirl and charge velocity. It is believed that a mechanism not previously appreciated is responsible for the improved lean burn capability of symmetrical twin tract porting. This mechanism involves a toroidal flow being created within the cylinder spilling over from the valve skirt. The turbulence in the toroid remains confined and better controlled and less kinetic energy is lost through friction with the cylinder walls.
An effect of the toroidal flow, which is believed to be the reason for the improved performance which has now been discovered, is that the fuel in the charge tends to be better atomised on account of the increased kinetic energy in the charge.
This explanation of the flow conditions created by symmetrical twin tract porting given above is consistent with the fact that in prior art proposals having a vertical tract and one lateral tract, better combustion occurred when fuel was injected into the vertical tract than into the lateral tract. The reason is that on account of the toroidal nature of the flow, the gases originating in one tract remain aligned with that tract and consequently the charge was stratified. Better combustion of leaner mixtures therefore occurred when a fuel concentration remained in the vicinity of the spark plug at the end of the compression stroke.
It is accordingly a preferred feature of the invention that in a fuel injected engine, fuel should be introduced into the cylinder through only one of the two tracts, namely the tract nearer the spark plug.
In the case of a carburetted engine, metering fuel in dependence upon the air flow through only one of the two intake tracts is not believed to be sufficiently accurate. However, in such an engine, the present invention, by providing access to both tracts from the same side of the engine, permits a one piece branched manifold to be employed to connect both tracts to a common carburettor. Of course, it is still possible to meter the fuel to only one tract by using a carburettor and in this case the port design of the engine also simplifies the manifold configuration. For example, one set of tracts may be supplied with air only through a first manifold and the other set of tracts may be supplied with a fuel/air mixture via a second manifold leading to a carburettor, the two manifolds having ganged throttles.